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Marine Mammals of the Gulf of Mexico

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Marine Mammals of the Gulf of Mexico Part Two THE BIOTA OF THE GULF OF MEXICO 62 MARINE MAMMALS IN THE GULF OF MEXICO: CURRENT KNOWLEDGE AND RECOMMENDATIONS FOR THEIR CONSERVATION Joel G. Ortega-Ortiz, Alberto Delgado-Estrella and Alejandro Ortega-Argueta INTRODUCTION The group known as “marine mammals” includes vertebrates of the mammalian class that spend most of their lives and obtain most of their food in the aquatic environment whether this is marine or fresh water. This group is not a taxonomic group in the true sense, but a collection of mammals of three orders: Carnivora (seals, sea wolves and walrus, known as pinnepeds), Sirenia (manatees and dugongs) and Cetacea (whales, dolphins and porpoise). In the Gulf of Mexico, 30 species of marine mammals have been documented: one carnivore species, one siren species and 28 cetacean species (Jefferson and Schiro 1997; Wursig et al. 2000). The only carnivore species that inhabited the Gulf in the contemporary period is the Caribbean monk seal (Monachus tropicalis). It became extinct by the middle of the 20th century due to hunting by the inhabitants of the Caribbean region (Villa-R. et al. 1985; LeBoeuf et al. 1986). The West Indian manatee (Trichechus manatus) is the only species of siren in the region. The cetacean species of the suborder Odontoceti (dolphins and whales with teeth) that have been documented in the Gulf of Mexico are: cachalot (Physeter macrocephalus), pygmy cachalot (Kogia breviceps), dwarf cachalot (K. sima), short-finned pilot whale (Globicephala macrorhynchus), pygmy killer whale (Peponocephala electra), Fraser dolphin (Lagenodelphis hosei), Risso dolphin (Grampus griseus), Clymene dolphin (Stenella clymene), striped dolphin (S. coeruleoalba), Atlantic spotted dolphin (S. frontalis), pan-tropical spotted dolphin (S. attenuata), spinner dolphin (S. longirostris), rough-toothed dolphin (Steno bredanensis), grampus whale (Orcinus orca), false killer whale (Pseudorca crassidens), pygmy grampus whale (Feresa attenuata), bottlenose dolphin (Tursiops truncatus), Gervais’ beaked whale (Mesoplodon europaeus), Blainville beaked whale (M. densirostris), Cuvier beaked whale (Ziphius cavirostris) and Sowerby beaked whale (M. bidens). In the cetacean suborder Mysticeti, that includes mustached whales, the following species have been documented in the Gulf: northern white whale (Eubalaena glacialis), blue whale (Balaenoptera musculus), common rorqual (B. physalus), tropical rorqual (B. edeni), northern rorqual (B. borealis), minke whale (B. acutorostrata) and humpback whale (Megaptera novaeangliae) (Jefferson and Schiro 1997; Würsig et al. 2000). DISTRIBUTION DISTRIBUTION OF THE WEST INDIAN MANATEE IN THE GULF OF MEXICO The West Indian manatee lives in shallow waters in marine zones, estuaries, lagoons, rivers and channels. It can live in both fresh and salt water. When it lives in marine environments it needs to be near freshwater for drinking purposes, and this can be spring water or in the discharge from rivers. Manatees can move over several hundred kilometers and their movement can be influenced by seasonal changes in water levels, by changes in salinity and water temperature in estuaries, changes in the food availability, and as a consequence of reproductive behavior (Colmenero R. and Hoz Z. 1986; Reynolds and Odell 1991). Driven by these factors, 63 the manatees can move between riverine environments towards the coast and vice versa. It is thought that the populations of manatees in the Gulf of Mexico move back and forth between coastal and palustrine habitats. There are two subspecies of the West Indian manatee in the Gulf of Mexico, the Florida (T. m. latirostris) and the Caribbean (T. m. manatus) (Rice 1998). The Florida manatee is resident along the Florida coastline, in bays and rivers south of the Crystal River. In summer, some individuals move north towards Georgia and to the west towards Louisiana. The historic distribution of the Caribbean manatee inside the Gulf of Mexico extends from Tamaulipas to Yucatan (Lefebvre et al. 2001). However, the distribution of this subspecies is fragmented as a result of overexploitation for meat and loss of habitat due to coastal development. The core of manatee populations in Mexico, in terms of population size, are: a small, dispersed population that inhabits several rivers in the state of Veracruz; a second population on the coast of Quintana Roo, mainly in the southern part; and the largest population, that inhabits the Grijalva and Usumacinta river basin. DISTRIBUTION OF CETACEA IN THE GULF OF MEXICO Seasonal and spatial distributions of cetaceans are influenced by physiographic and oceanographic characteristics, distribution of prey, distribution of predators and availability of suitable areas for breeding. In the Gulf of Mexico, water depth is the main environmental variable that influences distribution of cetaceans. Significant differences have been observed in the depth of water inhabited by the different species (Davis et al. 1998; Baumgartner et al. 2001, Davis et al. 2002; Ortega-Ortiz 2002). The majority of cetacean species inhabit oceanic waters more than 200 m deep. Only three species normally inhabit the neritic zone on the continental shelf: the bottlenose dolphin, the Atlantic spotted dolphin and the rough-toothed dolphin (Delgado-Estrella 1994; Jefferson and Schiro 1997; Wursig et al. 2000; Ortega-Ortiz 2002; Fulling et al. 2003). The spinner dolphin has been documented on the northern part of the Yucatan shelf (Jefferson 1995; Ortega-Ortiz 2002), however, this species is generally oceanic and its sighting near the coast could be associated with upwelling (as suggested by Merino 1997). The bottlenose dolphin is the only cetacean that inhabits coastal lagoons, river deltas, littoral zones and neritic and oceanic zones of the Gulf of Mexico. Morphological and genetic evidence suggests the existence of two ecotypes or parapatric populations of bottlenose dolphin, one coastal and the other neritic-oceanic (Hersh and Duffield 1990; Mead and Potter 1995; Hoelzel et al. 1998; Torres et al. 2003). Although there is not detailed delineation of the two distributional areas for the two ecotypes, a study of the Atlantic coast of the United States of America showed that all bottlenose dolphins observed up to 7.5 km from the coast were of the coastal ecotype, whereas all observed more than 34 km from the coast were of the neritic-oceanic ecotype (Torres et al. 2003). Coastal bottlenose dolphins are more frequently found inside coastal lagoons and near river deltas. It addition, dolphins use specific areas for feeding and others for breeding within coastal lagoons (Delgado-Estrella 2002). In the north of the Gulf, the neritic-oceanic ecotype of bottlenose dolphin has been seen in two different regions: on the continental shelf at depths of up to 150 m, and over the slope near the continental margin. In both regions, dolphins are more frequently observed in areas with highly variable temperatures and salinities suggesting that their distribution is associated with water mass fronts (Baumgartner et al. 2001). 64 The Atlantic spotted dolphin is generally found in waters between 20 and 200 m deep. They have occasionally been observed in oceanic waters near the continental margin (Mullin and Hansen 1999). Griffin and Griffin (2003) compared the distribution of bottlenose dolphin and Atlantic spotted dolphin in depths of < 20 m on the west coast of Florida. These authors observed that the Atlantic spotted dolphin was found in deeper waters, with higher salinity and clearer water than those that are inhabited by bottlenose dolphin. The oceanic cetaceans of the Gulf of Mexico are most frequently found in the continental slope and areas with greater concentrations of chlorophyll, such as in the cyclonic gyres and the areas of confluence between anticyclonic and cyclonic gyres (Biggs et al. 2000; Baumgartner et al. 2001; Davis et al. 2002; Ortega-Ortiz 2002). In addition to bottlenose dolphin, the species of that is more frequently found in the upper slope near the continental margin and at depths of 200- 750 m are the Risso dolphin and the short-finned pilot whale. Also on the upper slope (200-1,000 m), but not necessarily near the shelf margin, the cachalot, pygmy cachalot, dwarf cachalot, rough-toothed dolphin, and the spinner dolphin are frequently found. On the lower slope of the shelf (>1,000 m), the striped dolphin, beaked whales, and the pan-tropical spotted and Clymene dolphins are found (Davis et al. 1998; Baumgartner et al. 2001; Davis et al. 2002). Little information has been published on the distribution of cetaceans in the deep region (>2,000 m) in the Gulf of Mexico. The mustached whales are rare in the Gulf of Mexico. With the exception of the tropical rorqual, observations of mysticetes are perhaps due to individuals that have lost their way during their migration between the north Atlantic and the Caribbean Sea (Jefferson and Schiro 1997; Wursig et al. 2000). The tropical rorqual has been documented as occurring year-round in the Gulf of Mexico. The majority of the sightings of the tropical rorqual are on the shelf, very near the continental margin. It is possible that the Gulf is a portion of the distribution area of a small and dispersed population of this species (Jefferson and Schiro 1997). CURRENT CONDITION OF POPULATIONS With the exception of the Sowerby beaked whale, all species of marine mammals in the Gulf of Mexico are included in the list of species at risk according to the Norma Oficial Mexicana (NOM-059-ECOL-2001; D.O.F. 2002). In these standards, the Caribbean monk seal is considered extinct in the wild state, the the northern white whale and the West Indian manatee are considered in danger of extinction and the rest of the species are subject to special protection. CURRENT CONDITIONS OF WEST INDIAN MANATEE POPULATIONS There is little information on the current condition of manatee populations in the southern Gulf of Mexico. This is due, in part, to the difficulty of studying a species in the wild. In 1976 it was estimated that there could be as many as 5,000 manatees in Mexico (Heinsohn 1976), however due to hunting and habitat loss it is very probable that the population has decline considerably since then.
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